The present invention relates to a semiconductor switching device used, for example, for a converter in a power system to convert AC to DC and vise versa.
A conventional semiconductor switching device, such as described in Japanese Unexamined Patent Publication No. 33,001/1999, is shown in FIG. 26. Although a semiconductor switching device typically comprises semiconductor switching elements connected in series and parallel, only one of them is shown in FIG. 26. As shown in FIG. 26, a reverse-conducting diode 3 is connected to a semiconductor switching element 2 to produce a reverse-conducting switching element 1. Moreover, a snubber circuit 10 is connected to the reverse-conducting switching element 1. In a semiconductor switching device in which reverse-conducting switching elements 1 are connected in series and parallel, each reverse-conducting switching element 1 is called a series valve and each reverse-conducting switching element 1 with the snubber circuit 10 is called a stage.
In a snubber circuit of FIG. 26, 4 denotes the first diode, 5 denotes the first capacitor, 6 denotes the first impedance element, 7(7a, 7b) denotes the first Zener diode, 8(8a, 8b) denotes the first resistor, and 9(9a, 9b) denotes the first controlling semiconductor element. The first impedance element 6, the first controlling semiconductor elements 9, the first resistors 8, and the first Zener diode 7 comprise a non-linear circuit 16. In FIG. 6, moreover, 12 denotes a gate drive circuit to drive the semiconductor switching element 2, 13 denotes an input terminal to apply a control signal for the gate drive circuit 12, and 33 denotes a power supply for the gate drive circuit.
In the semiconductor switching device in which the stages as shown in FIG. 26 are connected in series, overvoltage may be applied to a semiconductor switching element 2 mainly by following three causes.
Firstly, overvoltage is caused by asynchronous turn on/off operation among the semiconductor switching elements. If a semiconductor switching element 2 turns on later than the others, overvoltage is applied to this semiconductor switching element, turning on late. If a semiconductor switching element 2 turns off earlier than the others, an overvoltage is applied to this semiconductor switching element, turning off early.
Secondly, overvoltage is also caused by inductance of circuits connected thereto. When the semiconductor switching elements turns off, variation in current generates an electromotive force across the inductance of the circuits. This electromotive force is added to the semiconductor switching elements, whereby each semiconductor switching element is charged with the overvoltage at the same time.
Thirdly, in a semiconductor switching device in which semiconductor switching elements are connected in series, leakage current of each semiconductor switching element probably varies so that divided voltage for each semiconductor switching element also varies. Therefore, some semiconductor switching elements are charged with higher voltage than the other semiconductor switching elements, that is, overvoltage.
When an overvoltage occurs and is applied to the semiconductor switching element 2 in the semiconductor switching device as shown in FIG. 26, electrical charge from this overvoltage flows through the first diode 4 into the first capacitor 5. If, thereby, the voltage across the first capacitor 5 exceeds a Zener voltage determined by the first Zener diode 7(7a, 7b), current is drawn through he first impedance element 6 and the first controlling semiconductor element 9(9a, 9b) so that the voltage on the first capacitor 5 is decreased to the Zener voltage and the semiconductor switching element 2 is protected from the overvoltage.
By the way, if a voltage equal to or just above the normal voltage for each stage is chosen as the Zener voltage of the Zener diode 9 (9a, 9b), a slight overvoltage easily exceeds the Zener voltage and current continuously flows through the first controlling semiconductor element 9 (9a, b) so that the first controlling semiconductor element 9 (9a, 9b) may be thermally destroyed. While, if voltage considerably higher than the normal voltage is chosen as the Zener voltage, unevenness in applied voltage among the stages, which is caused by uneven leakage current among the semiconductor switching elements 2 in the off state, is not compensated until the highest voltage among semiconductor switching elements 2 reaches this considerably high Zener voltage. Therefore, compensation to apply equal voltage for each semiconductor element 2 cannot be achieved.
Moreover, in case a semiconductor switching device is constructed from a large number of semiconductor switching elements 2 connected in series and parallel, a power supply 33 for a gate drive circuit 12 is required for each semiconductor element 2 so that the semiconductor switching device as a whole becomes complicated and manufacturing cost thereof rises.
As described above, electromotive force across the inductance of the circuits is generated by turn off of the semiconductor switching elements 2 and, thereby, current flows into the first capacitor 5. During the current flows into the first capacitor 5, power is also supplied from the power source to the first capacitor 5. Therefore, the current caused by the electromotive force of inductance and the current from the power source flowing together with the current are stored in the first capacitor 5 and lost at the snubber circuit 10.
By setting an upper limit voltage of the first capacitor 5, i.e., the Zener voltage of the Zener diode 7 (7a, 7b), above the normally applied voltage of each semiconductor switching element 2, the current from the power source toward the first capacitor 5, which originates from the electromotive force at the inductance, hardly flows. Thus, the term in which the current flows is shortened so that energy from the power source supplied to the first capacitor 5 is reduced.
As described above, by s fling the voltage of the first capacitor 5 higher than the normally applied voltage of ea h semiconductor switching element 2, loss in the snubber circuit 10 is reduced. However, if the voltage of the first capacitor 5 is set too high, protection for the semiconductor elements 2 becomes insufficient so that breakdown of the semiconductor elements 2 may be caused. In contrast, if voltage of the first capacitor 5, i.e., the Zener voltage of the Zener diode 7 (7a, 7b), is set too small, the voltage applied to each semiconductor element 2 easily exceeds the Zener voltage by a slight increase thereof due to an accident or the like in the power system and, thereby, current continuously flows through the Zener diode 7 (7a, 7b) so that the Zener diode 7 (7a, 7b) may be thermally destroyed.
Moreover, when a higher energy voltage is chosen to reduce loss in the snubber circuit 10, unevenly divided voltages among the semiconductor switching elements 2 in their off state hardly exceed this higher Zener voltage so that compensation to apply equal voltage for each semiconductor element 2 in off state cannot be achieved.
Furthermore, especially in a semiconductor switching device in which a large number of the semiconductor switching elements 2 are connected in series to convert very high voltage, a power supply 33 to supply adequate voltage for a gate drive circuit 12 of each semiconductor switching element 2 becomes complicated and costly.
Therefore, in the first aspect of the present invention, switching elements of self-quenching function are connected in series to constitute a bridge arm, and at least two bridge arms are connected in parallel to constitute a high voltage semiconductor switching device. Moreover, a snubber circuit comprising a first diode, a first capacitor and a non-linear circuit is connected in parallel to the respective semiconductor switching element. In the snubber circuit, the first diode and the first capacitor are connected in series, and the non-linear circuit is connected in parallel to the first capacitor. The non-linear circuit comprises a impedance element, a non-linear circuit element such as Zener diode, and a controlling semiconductor element and draws current through the controlling semiconductor element when applied voltage exceeds the Zener voltage of the Zener diode, and said Zener voltage is larger than and has minimum latitude to divided voltage of the semiconductor element under an accident in power system.
In the semiconductor switching device according to the first aspect of the present invention, overvoltage caused by asynchronous turn on/off among the semiconductor switching elements can be absorbed into the snubber circuit and, at the same time, loss at the snubber circuit can be reduced. Moreover, even if voltage of power source rises by an accident such as short circuit in a load connected thereto to raise divided voltage of the semiconductor switching element, the semiconductor switching element as well as the snubber circuit is protected and never be broken.
In the second aspect of the present invention, with considering our research showing that the rise in voltage of power source under the accident is 1.27 times at maximum, clamp voltage at which the non-linear circuit begins to draw the current is set to approximately 1.3 times larger than normal divided voltage of the semiconductor switching element.
In the non-linear circuit of the semiconductor switching device according to the third aspect of the present invention, cathode and anode of the first Zener diode are connected to collector and gate of the first controlling semiconductor element respectively, the first resistor is connected between gate and emitter of the first controlling semiconductor element, and impedance of the first impedance element or number of the first controlling semiconductor elements is adjusted with which loss at the first controlling semiconductor element owing to the current, which flows through the first controlling semiconductor element when divided voltage of the semiconductor switching element under an accident in power system exceeds the clamp voltage, does not exceed capacity of the first controlling element.
In the semiconductor switching device according to the third aspect of the present invention, even if one or more semiconductor switching element is broken so that divided voltage of the other semiconductor switching element rises and, in addition, some accident happens so that the voltage of power source rises, thermal heat at the first controlling semiconductor element can be reduced by enlarging impedance of the first impedance element to reduce current flowing into the non-linear unit or by increasing the number of the first controlling semiconductor elements to reduce thermal heat for each first controlling semiconductor element. Thereby, the first controlling semiconductor element is protected from to be broken.
In the semiconductor switching device according to the fourth aspect of the present invention, a second impedance element having resistance, a second Zener diode comprising Zener diodes and a second resister are connected in series and connected parallel to the semiconductor switching element. Moreover, a second controlling semiconductor element, which draws current when voltage applied to the semiconductor switching element exceeds Zener voltage of the second Zener diode, is provided in a manner such that collector and gate thereof are connected to cathode and anode of the second Zener diode respectively, and said second resistor is connected between gate and emitter thereof.
In the semiconductor switching element in which large number of the semiconductor switching elements are connected in series, the leakage current of each semiconductor switching element is uneven so that the normal divided voltage for the semiconductor switching element of larger leakage current becomes lower and the normal divided voltage for the semiconductor switching element of smaller leakage current becomes higher. However, in the semiconductor switching device according to the fourth aspect of the present invention, current is drawn through said circuit in accordance with raised normal divided voltage owing to the unevenness in the leakage current. Consequently, normal divided voltage of each semiconductor switching element is almost equalized regardless of individual characteristics thereof.
In the semiconductor switching device according to the fifth aspect of the present invention, the non-linear circuit comprises a first impedance element and non-linear units. A second impedance element connects high voltage side of the semiconductor switching element to connecting point between the non-linear units, or low voltage side of the semiconductor switching element to connecting point between the nonlinear units.
In the semiconductor switching element in which large number of the semiconductor switching elements are connected in series, the leakage current of each semiconductor switching element is uneven so that the normal divided voltage for each semiconductor switching element is also uneven. Therefore, the semiconductor switching device according to the fifth aspect of the present invention compensates this unevenness of the leakage current to equalize the normal divided voltage among the semiconductor switching elements. The normal divided voltage for the semiconductor switching element of larger leakage current is lower and the normal divided voltage for the semiconductor switching element of smaller leakage current is higher. The non-linear units detects this higher normal divided voltage and draw current therethrough additionally to the leakage current through the semiconductor switching element.
In the semiconductor switching device according to the sixth aspect of the present invention, the second impedance element connects high voltage side of the semiconductor switching element to connecting point between the non-linear units, and the second impedance element itself comprises a third diode and a third resistor connected in series.
In the semiconductor switching device according to the sixth aspect of the present invention, the third resistor controls and adjusts current flowing through the second impedance element to make the normal divided voltage more closer to the ideal equally divided voltage.
In the semiconductor switching device according to the seventh aspect of the present invention, the first Zener diode or the second Zener diode is selected to make the second clamp voltage determined by the non-linear unit, which is connected to the second impedance element with cathode of the Zener diode thereof, lower than the normal divided voltage of the semiconductor switching element.
In the semiconductor switching device according to the seventh aspect of the present invention, since the second clamp voltage or Zener voltage determined by the first Zener diode or the second Zener diode is lower than the normal divided voltage of the semiconductor switching element, the applied voltage of the semiconductor switching element higher than or closer to the equally divided voltage is detected so that compensating current is drawn through the second impedance element to make the normal divided voltage closer to the equally divided voltage.
In the semiconductor switching device according to the eighth aspect of the present invention, the second impedance element is not provided for at least one semiconductor switching element in which resistance or normal divided voltage in off state thereof is smaller than the other semiconductor switching elements.
In the semiconductor switching device according to the eighth aspect of the present invention, since necessity to flow the compensating current is small for the semiconductor switching element of smallest resistance or smallest normal divided voltage, the second impedance element there of can be omitted.
In the semiconductor switching device according to the ninth aspect of the present invention, the second impedance element comprises a third diode and a variable resistor. The variable resistor is adjusted with considering resistance of each semiconductor switching element or normal divided voltage of each semiconductor switching element to flow adequate compensating current which makes voltage of each semiconductor switching element closer to the equally divided voltage.
In the semiconductor switching device according to the tenth aspect of the present invention, a DC voltage converter is connected to the first capacitor with input terminals thereof and connected to power input terminals of a gate drive circuit with output terminals thereof. Voltage of the first capacitor is regulated by the DC voltage converter and, then, supplied to the gate drive circuit.
In the semiconductor switching device according to the eleventh aspect of the present invention, the second capacitor is connected in parallel to at least one non-linear circuit for supplying gate driving voltage. A DC voltage converter is connected to the second capacitor with input terminals thereof and connected to power input terminals of a gate drive circuit with output terminals thereof. Voltage of the second capacitor is regulated by the DC voltage converter and, then, supplied to the power input terminals of the gate drive circuit.
In the semiconductor switching device according to the twelfth aspect of the present invention, the first capacitor comprises series connected capacitors and each capacitor is connected to the respective non-linear unit in parallel. A DC voltage converter is connected in parallel to one or more capacitors of lower voltage side with input terminals thereof and connected to power input terminals of a gate drive circuit with output terminals thereof. Voltage of said one or more capacitors is regulated by the DC voltage converter and, then, supplied to the power input terminals of the gate drive circuit.
According to the thirteenth aspect of the present invention, the first diode of the above described semiconductor switching device is replaced with a power regeneration switch. For example, the power regeneration switch comprises a diode having the same forward direction as the first diode and a switching element connected in parallel thereto. By turning the switching element on following turn off of the semiconductor element and turning the switching element off before the semiconductor switching element turns on again, energy stored into the first capacitor through the diode of the power regeneration switch is regenerated to the power source through the switching element of the power regeneration switch.
In the semiconductor switching device according to the fourteenth aspect of the present invention, a large Qrr diode, in which reverse recovery charge is 10 percent or more of forwardly flowed charge when the semiconductor switching element turns off, is used as the first diode. By using the large Qrr diode, i.e. diode in which reverse recovery charge is large, charge Qrr as same as charge Qin, which flowed into the first capacitor when the semiconductor switching element turned on, is regenerated to the power source during reverse recovery period thereof.
In the semiconductor switching device according to the fifteenth aspect of the present invention, a diode-on delay device is connected to the first diode in parallel.
In case the semiconductor switching device comprising semiconductor switching elements connected in series and the semiconductor switching elements turns on asynchronously, current flows through the fist diode and the first capacitor connected to the semiconductor switching element of still off state toward the semiconductor switching element of on state. Afterward, the semiconductor switching element turns on lately, reverse current occurs in the first diode connected thereto so that the first diode may be broken.
With the diode-on delay device, beginning of conduction at the first diode is delayed so that all the semiconductor switching elements turns on before current starts to flow through the first diode and first capacitor. Thereby, current bypassing the semiconductor switching element of late turn on does not flows so that clue to the reverse current does not happens. Accordingly, safe and reliable semiconductor switching device can be obtained regardless of the asynchronous turn on among the semiconductor switching elements.